Catheters are used increasingly as a means for delivering diagnostic and therapeutic agents to internal sites within the human body that can be accessed through the various of the body's lumen systems, particularly through the vasculature. A catheter guidewire is used for guiding the catheter through the bends, loops, and branches forming the blood vessels within the body. One method of using a guidewire to direct the catheter through the torturous paths of these systems of lumen involves the use of a torqueable guidewire which is directed as a unit from a body access point such as the femoral artery to the tissue region containing the target site. The guidewire is typically bent at its distal end, and may be guided by alternately rotating and advancing the guidewire along the small vessel pathway to the desired target. Typically the guidewire and the catheter are advanced by alternately moving the guidewire along a distance in the vessel pathway, holding the guidewire in place, and then advancing the catheter along the axis of the guidewire until it reaches the portion of the guidewire already advanced farther into the human body.
The difficulty in accessing remote body regions such as the body's periphery or the soft tissues within the body, such as the brain and the liver, are apparent. The catheter and its attendant guidewire must be both flexible, to allow the combination to follow the complicated path through the tissue, and yet stiff enough to allow the distal end of the catheter to be manipulated by the physician from the external access site. It is common that the catheter is as long as a meter or more.
The catheter guidewires used in guiding a catheter through the human vasculature have a number of variable flexibility constructions. For instance, U.S. Pat. Nos. 3,789,841; 4,545,390; and 4,619,274 show guidewires in which the distal end section of the wire is tapered along its length to allow great flexibility in that remote region of the guidewire. This is so, since the distal region is where the sharpest turns are encountered. The tapered section of the wire is often enclosed in a wire coil, typically a platinum coil, to increase the column strength of the tapered wire section without significant loss of flexibility in that region and also to increase the radial capacity of the guidewire to allow fine manipulation of the guidewire through the vasculature.
Another effective guidewire design is found in U.S. Pat. No. 5,095,915 which shows a guidewire having at least two sections. The distal portion is encased in an elongated polymer sleeve having axially spaced grooves to allow increased bending flexibility of the sleeve.
Others have suggested the use of guidewires made of various super-elastic alloys in an attempt to achieve some of the noted functional desires.
U.S. Pat. No. 4,925,445, to Sakamoto et al., suggests the use of a two-portion guidewire having a body portion relatively high in rigidity and a distal end portion which is comparatively flexible. At least one portion of the body and the distal end portions is formed of super-elastic metallic materials. Although a number of materials are suggested, including Ni--Ti alloys of 49 to 58% (atm) nickel, the patent expresses a strong preference for Ni--Ti alloys in which the transformation between austenite and martensite is complete at a temperature of 10.degree. C. or below. The reason given is that "for the guidewire to be useable in the human body, it must be in the range of 10.degree. to 20.degree. C. due to anesthesia at a low body temperature." The temperature of the human body is typically about 37.degree. C.
Another document disclosing a guidewire using a metal alloy having the same composition as a Ni--Ti super-elastic alloy is WO91/15152 (to Sahatjian et al. and owned by Boston Scientific Corp.). That disclosure suggests a guidewire made of the precursor to the Ni--Ti elastic alloy. Super-elastic alloys of this type are typically made by drawing an ingot of the precursor alloy while simultaneously heating it. In the unstressed state at room temperature, such super-elastic materials occur in the austenite crystalline phase and, upon application of stress, exhibit stress-induced austenite-martensite crystalline transformations which produce nonlinear elastic behavior. The guidewires described in that published application, on the other hand, are said not to undergo heating during the drawing process. The wires are cold drawn and great pain is taken to assure that the alloy is maintained well below 300.degree. F. during each of the stages of its manufacture. This includes the step of grinding the guidewire to form various of its tapered sections.
U.S. Pat. No. 4,665,906 suggests the use of stress-induced martensite (SIM) alloys as constituents in a variety of different medical devices. Such devices are said to include catheters and cannulas.
U.S. Pat. No. 4,969,890 to Sugita et al., suggests the production of a catheter having a main body fitted with a shape memory alloy member, and having a liquid injection means to supply a warming liquid to allow the shape memory alloy member to recover its original shape upon being warmed by the fluid.
U.S. Pat. No. 4,984,581, to Stice, suggests a guidewire having a core of a shape memory alloy, the guidewire using the two-way memory properties of the alloy to provide both tip-deflecting and rotational movement to the guidewire in response to a controlled thermal stimulus. The controlled thermal stimulus in this instance is provided through application of an RF alternating current. The alloy selected is one that has a transition temperature between 36.degree. C. and 45.degree. C. The temperature 36.degree. C. is chosen because of the temperature of the human body; 45.degree. C. is chosen because operating at higher temperatures could be destructive to body tissue, particularly some body proteins.
U.S. Pat. No. 4,991,602 to Amplatz et al., suggests a flexible guidewire made up of a shape memory alloy such as the nickel-titanium alloy known as nitinol. The guidewire is one having a single diameter throughout its midcourse, is tapered toward each end, and has a bead or ball at each of those ends. The bead or ball is selected to allow ease of movement through the catheter into the vasculature. The guidewire is symmetrical so that a physician cannot make a wrong choice in determining which end of the guidewire to insert into the catheter. The patent suggests that wound wire coils at the guidewire tip are undesirable. The patent further suggests the use of a polymeric coating (PTFE) and an anticoagulant. The patent does not suggest that any particular type of shape memory alloy or particular chemical or physical variations of these alloys are in any manner advantageous.
Another catheter guidewire using Ni--Ti alloys is described in U.S. Pat. No. 5,069,226, to Yamauchi, et al. Yamauchi et al. describes a catheter guidewire using a Ni--Ti alloy which additionally contains some iron, but is typically heat-treated at a temperature of about 400.degree. to 500.degree. C. so as to provide an end section which exhibits pseudo-elasticity at a temperature of about 37.degree. C. and plasticity at a temperature at a temperature below about 80.degree. C. A variation is that only the end portion is plastic at the temperatures below 80.degree. C.
U.S. Pat. No. 5,171,383, to Sagae, et al., shows a guidewire produced from a super-elastic alloy which is then subjected to a heat treatment such that the flexibility is sequentially increased from its proximal portion to its distal end portions. A thermoplastic coating or coil spring may be placed on the distal portion of the wire material. Generally speaking, the proximal end portion of the guidewire maintains a comparatively high rigidity and the most distal end portion is very flexible. The proximal end section is said in the claims to have a yield stress of approximately five to seven kg/mm.sup.2 and an intermediate portion of the guidewire is shown in the claims to have a yield stress of approximately 11 to 12 kg/mm.sup.2.
Published European Patent Application 0,515,201-A1 also discloses a guidewire produced at least in part of a superelastic alloy. The publication describes a guidewire in which the most distal portion can be bent or curved into a desired shape by a physician immediately prior to use in a surgical procedure. Proximal of the guide tip, the guidewire is of a superelastic alloy. Although nickel-titanium alloys are said to be most desirable of the class shown in that disclosure, no particular physical description of those alloys is disclosed to be any more desirable than another.
Published European Patent Application 0,519,604-A2 similarly discloses a guidewire which may be produced from a superelastic material such as nitinol. The guidewire core is coated with a plastic jacket, a portion of which may be hydrophilic and a portion of which is not.
Examples of Ni--Ti alloys are disclosed in U.S. Pat. Nos. 3,174,851; 3,351,463; and 3,753,700.
None of these disclosures suggest the guidewire composition or configuration described below.